Obsidian is natural volcanic glass, an igneous rock formed by the rapid cooling of felsic lava. Obsidian has its glass-like nature because it is practically pure silica, the basic component of glass. It's black color is due to the presence of small amounts of iron. Obsidian may be speckled with white dots of cristobalite, like in this sample, and the rock is then known as 'snowflake obsidian'. Actual size of this polished face is 9 x 13 cm.